Identification and tracking of surface moving targets is coming to a forefront of importance. Such tracking allows identification of vehicles that stop at locations suitable for the planting of Improvised Explosive Devices (IEDs), and ideally would also show the dismounted individual and his location. Tracking of surface moving targets can be performed by radar systems. Radar systems are widely known. Early radar systems used vacuum tubes in analog circuits, and were subject to reliability problems. In particular, the vacuum tubes would age with time, and the operating parameters of the radar system would vary on an almost continuous basis, requiring constant realignment. When left in operation, the many vacuum tubes of a radar system would give rise to frequent failures attributable to normal aging and often excessive heat. If turned off when not in use, the thermal cycling would often result in tube failures at each turn-on, requiring trouble-shooting and maintenance.
With the advent of solid-state devices, some of the low-frequency, low-power functions of a radar system could be converted away from tubes. This often provided a major improvement in reliability. The introduction of integrated circuits gave impetus for their use in the low-frequency, low-power sections of a radar system, and also gave impetus to the then-nascent field of digital signal processing.
Digital signal processing gained dominance in the field of radar signal processing. Advanced or special-purpose integrated circuits were developed to handle high frequencies. Solid-state devices have been limited, however, in their ability to handle the large amounts of power required for radar transmissions. This problem has been partially solved by dividing the power to be transmitted among a plurality of solid-state amplifiers, each of which provides transmit power to one or a few antenna elements of an active array antenna. Examples of such solid-state amplifiers appear in U.S. Pat. Nos. 4,601,106 and 4,641,107, issued Feb. 3, 1987 to Kalokitis; U.S. Pat. No. 4,780,685 issued Oct. 25, 1988 to Ferguson; and U.S. Pat. No. 4,965,530 issued Oct. 23, 1990 to Katz. Solid-state amplifiers are often found embedded within transmit-receive (T/R) modules. Such TR modules are described, for example, in U.S. Pat. No. 5,017,927, issued May 21, 1991 to Agrawal et al. The use of many such these T/R modules in a limited space gives rise to heat removal or temperature problems, and an art, exemplified by U.S. Pat. No. 6,469,671, issued Oct. 22, 2002 to Pluymers et al., has arisen to configure such systems for convenient heat removal.
The design of radar systems involves a complex tradeoff among many factors, among which are system complexity, cost, weight, performance, and reliability. Some of the problems, issues and considerations challenging the design of radars in various contexts are described or addressed in many publications, as for example in U.S. Pat. No. 4,885,590 issued Dec. 5, 1989 to Hasan; U.S. Pat. No. 5,103,233 issued Apr. 7, 1992 to Gallagher et al.; U.S. Pat. No. 5,151,702, issued Sep. 29, 1992 to Urkowitz; U.S. Pat. No. 5,157,403 issued Oct. 20, 1992 to Urkowitz; U.S. Pat. No. 5,309,161 issued May 3, 1994 to Urkowitz et al.; U.S. Pat. No. 5,343,208 issued Aug. 30, 1994 in the name of Chesley; U.S. Pat. No. 5,376,939 issued Dec. 27, 1994 to Urkowitz; U.S. Pat. No. 5,414,428 issued May 9, 1995 to Gallagher et al.; U.S. Pat. No. 5,440,311 issued Aug. 8, 1995 to Gallagher et al.; U.S. Pat. No. 5,481,270 issued Jan. 2, 1996 to Urkowitz et al.; U.S. Pat. No. 6,084,540 issued Jul. 4, 2000 to Yu; U.S. Pat. No. 6,184,820 issued Feb. 6, 2001 to Kratzer; U.S. Pat. No. 6,483,478 issued Nov. 19, 2002 to Yu; U.S. Pat. No. 6,639,546 issued Oct. 28, 2003 to Ott et al.; U.S. Pat. No. 7,081,848 issued Jul. 25, 2006 to Adams; and U.S. Pat. No. 6,861,974 issued Mar. 1, 2005 to Poe et al. The problems associated with radar system design are more numerous and complex than might be thought. As an example of problems which are not signal processing problems, U.S. Pat. No. 6,995,638, issued Feb. 7, 2006 in the name of Smith et al. describes a structural augmentation arrangement which is intended to aid in maintaining reliability attributable to physical flexure or movement between a transmitter and an associated antenna. Among other problems associated with radar system design are that the beamformers (if any), circulators (if any), T/R modules, and filters (if any) associated with each elemental antenna of the antenna array tend to be physically large. It is very desirable to be able to set the inter-antenna-element spacing of an antenna array based on operational factors such as operating frequency, beam width, sidelobe level, grating lobes, and the like. The large size of the T/R modules tends to make selection of an appropriate inter-antenna-element spacing difficult. This problem has been addressed by selecting an appropriate inter-antenna-element spacing, and in a related fashion, by feeding groups of antenna elements in common.
Many advanced radar systems rely on a plurality of antenna array elements with associated T/R modules. Each T/R module includes circuitry that provides an analog modulation of an RF signal's phase and amplitude characteristics. After these modulations are performed, an RF beamformer sums the module's individual signals to form a beam with directional gain. These analog modulation devices and analog beamformers are expensive, require considerable space, and may require cooling. If multiple simultaneous beams are required, a plurality of these circuits is needed within the same space, compounding the shortcomings of the architecture. Additionally, to control these analog circuits, multi-bit digital control signals must be sent to each circuit at a specified rate. This requires a computational source to generate the signals and a network to distribute them in a timely manner.
In some areas of conflict such as in the deserts of Iraq, flat terrain is well adapted to the use of airborne synthetic aperture radar (SAR) systems, so that a synthetic aperture radar system can scan a broad region and illuminate most targets in that region. Moving target indication (MTI) or Doppler processing can identify those targets which are moving, and can also determine the rate of motion, so as to be able to distinguish between a vehicle moving at, for example, 20 meters per second (m/sec) and a dismounted human, who might move at ½ m/sec.
In other areas of conflict, as in Afghanistan, the use of ground moving target indicating radar is made difficult by the mountainous terrain, which shadows much of the target region of interest.
Another problem associated with ground moving target indicating radar lies in the need for maximizing the area of coverage as much as possible, so that fewer ground moving target indicating radar systems are required overall. Maximizing the area of coverage tends to increase the time between successive “looks” at the target, which the result that, while spatial coverage may cover the desired area, temporal coverage suffers, or vice versa. That is to say, that a time of twenty or thirty seconds between successive “looks” at a particular target may allow stops of a vehicle or dismount to go unnoticed. Additional problems are that conventional GMTI systems use a “side looking” radar configuration where the search region is observed, at a distance, from a single side of the radar aircraft, at a low beam grazing angle. The aircraft performs this search while flying along one leg of a “race-track” flight pattern. In order to stay on station, the aircraft must turn 180° and fly along a second parallel leg of the race-track pattern. This continues throughout the radar mission. However, while the aircraft performs its 180° turns, it must cease to perform its radar mission, resulting in missed detection opportunities.
Improved or alternative ground or surface moving target indicating radar arrangements are desired.